1. Field of the Invention
The present invention relates to the measurement of color misconvergence of color video displays.
2. State of the Art
Most current video display devices are color displays. In the case of a color CRT, phosphors of different primary colors (i.e., RGB) are excited by separate electron beam guns, the intensities of excitation of the respective colors being calculated to produce a particular resultant color at a particular color pixel as light from the different color phosphors combine. When only one color gun is turned on for a particular pixel, that pixel is perceived as having that color. When R, G and B color guns are all turned on at equal intensities for a particular pixel, that pixel is perceived as white. By varying the intensities of the color guns in combination at a particular pixel, any of a very large number of colors may be produced at that pixel.
By scanning the color guns of a color CRT across the face of the CRT tube in a raster pattern, pixels throughout the viewing area of the CRT may be "painted" various colors. For proper color reproduction, the three separate color guns must act precisely in concert. The failure of the three separate color guns to act precisely in concert is referred to as "misconvergence," i.e., the failure of the electron beams to converge at a single color pixel, and results in the wrong color being displayed. Misconvergence may have both a horizontal component and a vertical component.
Good convergence is of paramount importance for a high-quality color video display. Misconvergence must therefore be tested and corrected for. Two different types of testing are performed, evaluation testing and production testing. During evaluation testing, misconvergence measurements are performed over an extended period of time under various operating conditions. In this manner, test results indicative of expected convergence performance over the life of the color video display may be obtained.
A conventional test setup for obtaining misconvergence measurements is shown in FIG. 1. A test computer includes a CPU, a monitor and a camera module disposed opposite a Unit Under Test (UUT). The computer is equipped with a video display card coupled to the UUT and an image capture card coupled to the camera module. A test image is displayed on the UUT, the display being imaged by the camera module. Images from the camera module are captured by the image capture card and processed by the CPU to obtain misconvergence measurements, which may then be displayed on the monitor for viewing by an operator.
The camera module may include a color CCD camera or a photodiode or a monochrome CCD camera. One color camera system that has been used for obtaining misconvergence measurements is the Minolta CC100 CRT convergence meter. Using such a system, information from each color signal is separated and a misconvergence result is calculated, e.g., by determining the positions of the individual color signals in the time domain in one frame. A similar approach may be used in the case of a photodiode sensor. Another prior-art method of obtaining misconvergence measurements uses a monochrome CCD camera and two patterns, Pattern 1 and Pattern 2. For Pattern 1, one color at a time is flickered on the screen, requiring three image capture operations to obtain all the required information. Pattern 2 is a grid pattern having red, green and blue portions. Information for each color is determined from prior knowledge of the three color portions. Using the information for each color, misconvergence results are calculated.
These prior-art methods suffer from various disadvantages. If a color CCD camera is used, color crosstalk produces measurement errors. If multiple images taken at different instants of time are used, measurement errors occur as a result of video/image jitter. In the case of a color grid display, Red, Green and Blue data are taken from different points of the image, such that the result calculated does not relate to the actual point of measurement, and the effect of lens distortion on the image can be significant.
Accordingly, there is needed an improved method of obtaining misconvergence measurements that overcomes the foregoing disadvantages.